In wireless communications, digital data is transmitted over the air via analog waveforms. The binary information is mapped into complex-valued modulated symbols representing one or more bits, transmitter (TX) pulse shaping filtering is applied, and the signal is up-converted to radio frequency (RF). At a receiver—also referred to in the art as user equipment—the signal is down-converted, receiver (RX) filtered, sampled, and demodulated. In the absence of channel impairments and if the sampling instant is chosen properly, the data may be recovered perfectly.
In practical systems, the timing of the TX and RX chains is not firmly synchronized, whereby the proper sampling instant is not known in advance. Rather, it depends on the transmission conditions, such as the propagation and processing delays. In addition, the radio channel and the receiver circuitry add noise and interference to the received signal, whereby the Signal to Interference and Noise Ratio (SINR) is degraded. In order to recover the signal with the highest quality possible, the choice of the sampling instant is important.
In order to provide the best sampling instant in a stable and robust manner, the RX clocking reference is typically locked to the TX frequency, as observed at the receiver, in a process known as Automatic Frequency Control (AFC). In addition, the phase of the sampling clock may be adjusted so as to provide the optimal sampling instant. Receiver frequency and time synchronization are well known in the art.
In typical implementations, the frequency and timing correction measures are applied to solve their respective tasks in a disjoint manner. The AFC is used to remove as much of the frequency offset as possible, by tuning the frequency reference or applying de-rotation. The usual criterion is to minimize the magnitude or mean squared error (MSE) of the residual frequency error after the correction.
Modern wireless broadband systems, such as HSPA Evolved in 3GPP, require high effective receiver SINR values, e.g., over 25 dB, to successfully receive data at the peak system rates. This requires very precise estimation and correction of timing errors, on the order of 1/64-th of a chip (4 ns) in HSDPA. Regardless of the methods used to estimate and correct the errors, practical estimation with sufficient quality requires the underlying process to be relatively stable over an extended period.
To minimize timing errors, and to track multipath signal components over time for calculating generalized RAKE (G-RAKE) finger placement delays, the relative temporal offsets of received multipath signal components are tracked in a path delay profile. In practice, this tracking is typically necessary even when the terminal is not physically moving, since the observed delay profile still drifts. When the path delay profile drifts over time, the positions of the RAKE or G-RAKE fingers (or channel equalization filter taps) must be updated in order to avoid losing signal energy, which implies shifting their positions. This means that, for example, the parameter filtering processes associated with the RAKE fingers must be reset or re-initialized to account for the new positions, and possible inter-finger distances. This causes additional computational load in the finger management circuitry, reducing the performance of the reception algorithms, as well as increasing power consumption.